1. Technical Field
The present invention relates to a metal paste sealing material for hermetically sealing, e.g., a tuning fork type or thickness shear vibration mode crystal resonator element in a package in a quartz crystal device such as a crystal unit, a crystal resonator, a crystal oscillator, a crystal filter, or a crystal sensor, and a quartz crystal device sealed using the metal paste sealing material and a method for sealing the quartz crystal device.
2. Related Art
Demands for smaller, thinner piezoelectric devices have been increasing, and many surface mount piezoelectric devices, which are suitable for mounting onto circuit boards and the like, have hitherto been used.
In surface mount piezoelectric devices, a configuration is employed in which a piezoelectric resonator element is sealed in a package made of an insulating material such as ceramic.
With a package in this former configuration, a piezoelectric resonator element is mounted in a cavity where thin plates made of ceramic materials are laminated on a box base, and a lid is hermetically bonded to the base so that the piezoelectric resonator element is sealed.
Regarding bonding of a base and a lid, in the case of a lid made of metal, a method of placing a metal seal ring on the top end surface of the base and seam-welding the lid to the seal ring (e.g., JP-A-2000-223606) and a method of placing a brazing metal material on the top end surface covered with a metal film of a base and heating and melting the material (see, e.g., JP-A-11-307661) are known.
In the case of a lid made of a ceramic or glass material, a method of heating and melting low-melting glass or resin placed between a base and a lid (see, e.g., JP-A-2001-244772) and a method of forming a metallized layer on the lower surface of a lid and melting an eutectic solder so that the lid is bonded to a ceramic case (see, e.g., JP-A-9-36690), and a method of heating and melting an eutectic metallic film layer formed in a bonding portion of a lid (see, e.g., JP-A-54-78694) are known.
However, there is a possibility that a gas produced from low-melting glass and high heat of seam-welding decrease or degrade frequency characteristics of a crystal resonator element.
Low-melting glass contains lead in many cases, and therefore may be not desirable in terms of a possibility of adversely affecting the environment.
To address these points, there has been proposed a crystal unit with a structure having a reduced size and a thinner thickness, in which a crystal plate having a crystal resonator element and an outer frame integrated therein is formed and substrates serving as a base and a cover are bonded on and under the crystal plate.
To hermetically seal such a structure, for example, a method of anode-bonding of metallic layers, which are formed on the upper and lower surfaces of an outer frame integrated with a crystal unit, with a cover and a case made of glass is known (see, e.g., JP-A-2000-68780).
A method in which surfaces to be mutually bonded of a piezoelectric plate and a substrate that have been subjected to mirror polishing are cleaned by applying ultraviolet rays or oxygen plasma, and the surfaces are bonded together using hydrogen bonding of —OH base by absorbing moisture is also known (see, e.g., JP-A-7-154177).
A method of putting together a bond surface having an Au film thereon of a piezoelectric substrate and a bond surface having an Ag film formed thereon of a protective substrate after plasma treatment and applying pressure and heating the surfaces to achieve diffusion bonding is also known (see, e.g., International Patent Publication No. WO 00/76066 pamphlet).
Further, a surface acoustic wave device formed by applying pressure to a metallic film formed on the main surface of a piezoelectric substrate having an interdigital transducer (IDT) formed thereon and a metallic film formed on the main surface of a base substrate, after activating the surfaces by ion beam and plasma irradiation, to directly bond together the surfaces is known (see, e.g., JP-A-54-78694).
Recently, there have been proposed methods that, in a container to contain electronic components such as piezoelectric resonators, hermetically seal a bonding portion of a box-type container formed of a ceramic material and a cover made of metal using a paste-like sealing member composed of metallic fine particles having an average particle size of 1 to 100 nm and a dispersion material such as amine (see, e.g., JP-A-2005-317793 and JP-A-2005-31-7794).
A method described in JP-A-2005-317793 applies a sealing member to a container or a cover by screen printing or an inkjet method and heating both the container and the cover in an atmosphere of 250° C. or less to bond together them.
A method described in JP-A-2005-317794 makes a paste-like sealing member become a sheet-like one and, using this one as a cover, bonds together the cover and the container to achieve a hermetic seal.
A piezoelectric device such as a crystal unit is also known.
In this device, a crystal plate are bonded with upper and lower plates with a metallic fine particle paste bonding material containing metallic fine particles that has an average particle size of 1 to 100 nm, the surfaces of which are coated with organic films, and which are dispersed in an organic solvent (see, e.g., JP-A-2006-186748).
The paste bonding material hermetically seals a piezoelectric device by metal-to-metal bonding between metallic fine particles and between metallic films that melts the organic films by heating at about 200° C. to make the metallic fine particles melt with each other, that is, to sinter them.
However, in a pasty sealing member or bond material made of the above-described metallic fine particles, nanoparticles having an average particle size of 100 nm or less are used as the metallic fine particles.
To prevent aggregation of the nanoparticles, a relatively large amount of organic solvent is required as the dispersing agent.
Therefore, gas may be generated from the organic solvent during sealing and remain inside a package, or gas may be generated over time from the organic solvent remaining in a bonding portion, causing a possibility of deteriorating the quality and frequency characteristics of a piezoelectric resonator element.
In general, organic solvent is difficult to handle, and therefore one problem with a sealing member containing a large amount of organic solvent is that the sealing member is not suitable for mass production and requires careful handling for its storage and transportation.